Conventional catalyst reactivation or regeneration processes that treat catalysts with a reduced catalytic activity due, at least in part, to deposition of coke on catalyst surfaces involve removal of that coke, generally by contacting such catalysts at high temperature (e.g. at least 450 degrees Celsius (° C.) for an ethanol dehydration catalyst and at least 650° C. for a fluid catalyst cracking (FCC) catalyst) with air or another oxygen-containing gas The conventional catalyst reactivation processes do not provide enough heat to drive the endothermic dehydrogenation reaction. Therefore, supplemental fuel must be added to some processes. The supplemental fuel further deactivates the catalyst in a catalytic dehydrogenation process using a gallium-platinum catalyst on alumina or alumina silica support, i.e., catalysts used in catalytic dehydrogenation of ethane, propane, butane, isobutane, butene, and ethylbenzene. Those who practice alkane dehydrogenation, especially propane dehydrogenation (PDH) understand that if enough heat is not provided to drive the endothermic reaction, alkene production decreases to a point where process economics dictate additional heat sources be added to drive the reaction.
During the regeneration process, the dehydrogenation activity is damaged due to the combustion of an external fuel source. The distribution of the fuel source is critical to achieve maximum combustion of the external fuel source on the catalyst as well as minimize any potential deactivation of the catalyst due to uneven distribution of fuel.
One constraint is that the distributor pipes themselves cannot block a high percentage of the combustor open area or the combustor will flood at this level or the catalyst will not be able to backmix and form a dense bed. For example, in one currently available design the distributor pipes block ˜26% of the open area. The base superficial velocity is 3.5-4 ft/s at this level in the combustor. With the blockage, the actual velocity will be 4.7-5.4 ft/s which are below the maximum of 8.0 ft/s where the catalyst will not be able to flow downward.
The fuel is often injected at ambient temperature which causes it to heat up as it transverses the distributor pipe within the bed which is operated at 680-800° C. and preferably from 700-770° C. As the fuel heats up, as it traverses a given pipe distributor, the gas density decreases which leads to maldistribution. The first part of the pipe will release more mass of fuel than the portion of the pipe that allows the gas to be in the vessel the longest. Therefore, maldistribution occurs. The maldistribution may cause portions of the combustor to have more stoichiometric fuel than air locally which means the fuel will have to mix with additional air and/or catalyst prior to combusting. This fuel contacting of the catalyst at high temperature results in catalyst deactivation that is not desirable.